Expandable polymer particles containing a polymer of a vinylarene monomer, such as polystyrene, are commercially important products that find use in many applications. These applications include thermal insulation and packaging material. In these applications there are also grades being used that are flame-retardant.
The polymer particles are turned into their eventual form by expansion and molding of the particles. It is desirable that the polymer particles should be rapidly foamable, i.e. that upon expansion low densities can be achieved in a short period of time. However, it was found that ways to improve the expandability of the polymer particle often have an adverse effect on particular properties of the final foamed product.
In U.S. Pat. No. 4,525,484 for example, it is taught that when the expandability of polymer particles is improved by the use of conventional low molecular weight plasticizers, this has the disadvantage that when the pre-foamed particles are foamed to give foam moldings, the plasticizers adversely affect the dimensional stability of the moldings. It was furthermore found in U.S. Pat. No. 4,525,484 that improving the expandability of certain polymer particles by using chain transfer agents is only adequate under specific conditions, since the process otherwise results in a polymer which has very high residual monomer content, shrinks greatly on foaming and on sintering give foam moldings having poor dimensional stability. U.S. Pat. No. 4,525,484 therefore discloses a preferred process wherein use is made of styrene oligomers to order to improve the expandability. However, a major drawback of these compounds is that they are expensive and not food-approved, which makes the resulting polymer particles unsuitable for use in e.g. food-packaging applications.
Furthermore, U.S. Pat. No. 4,525,484 teaches that it is essential for its polymer particles showing improved expandability to have a steep high molecular weight flank of the molecular weight distribution curve.